The present invention relates to a digital television special effect waveform generator which generates a gate signal used for inserting one video signal into another video signal.
In a prior art television special effect waveform generator, such a gate signal for composing two video signals is produced by slicing a fundamental wave generated in an analog technique by a fading level. In this case, it is possible to obtain a good quality insertion of the video signal by applying the slice level, i.e., the fading level, with a soft width corresponding to the area where two video signals are overlapped. For instance, when two video signals are combined through a circle on a TV screen, parabolic waves in the horizontal and vertical scanning periods are used as the fundamental wave. In the prior art, the fundamental wave such as the parabolic wave is produced by an analog technique or by a partial digital technique adopting a counter circuit or an arithmetic circuit. However, even when the partial digital technique is employed, the basic process for producing the fundamental wave is not renovated to a large extent in comparision with the conventional analog technique.
Furthermore, the prior art analog technique has disadvantages that when a minute fundamental wave is produced, it is easily distorted by noise. In addition, since the quality of the produced fundamental wave, especially that of the leading and trailing edges of the wave, is dependent on the characteristics of the components contained in the prior art generator, the regulation or adjustment thereof is complicated and troublesome, and the stability thereof is hardly sufficient. Moreover, in the situation where the special effect waveform is produced under the control of a computer as has become common recently, it is necessary for the special effect waveform generator to interface directly with the computer.